Apparatus for analyzing relaxation spectra and resonances in materials by thermal stimulated recovery

ABSTRACT

The present invention relates to an apparatus which is designed to employ thermal stimulated processes for analyzing relaxation spectra and resonances in materials. The invention is characterized in that at least two coupled excitation fields are applied to the sample of material analyzed along with a programmed temperature variation, to deconvolute during the thermally stimulated recovery stage the global deformation resulting from the excitation stage. In other words, this invention is designed to obtain one by one the individual and elementary relaxation motions responsible for the global deformation, whether these elementary internal motions have a mechanical, electrical or magnetic origin. Moreover, the relaxation spectra for the motions resulting from the coupling between mechanical, electrical and/or electromagnetical excitations are obtained at the same time and are interrelated.

CROSS-REFERENCE INFORMATION

This is a continuation-in-part application of co-pending applicationSer. No. 803,791 filed Dec. 6, 1991, now U.S. Pat. No. 5,152,607 whichis a continuation of application Ser. No. 470,782 filed Jan. 26, 1990,now abandoned.

FIELD OF THE INVENTION

The present invention relates to an apparatus which uses thermalstimulated processes for analyzing relaxation spectra and resonances inmaterials by application of programmed disturbances of the internalstate of the material, during the excitation stage, and by the study ofthe thermally stimulated recovery of these deformations, during thereturn to equilibrium stimulated by heating.

BACKGROUND OF THE INVENTION

Thermally stimulated processes are documented in the literature (Chapter10 in the ACS Book Polymer Characterization, "Characterization ofPolymers by Thermally Stimulated Current Analysis and Relaxation MapAnalysis Spectroscopy, by J. P. Ibar, et al., Polymer CharacterizationAdvances in Chemistry Series No. 227, Edited by Clara D. Craver andTheodore Provder; and for the TSCR: Chapter in the ACS Book PolymerCharacterization. "Thermally Stimulated Creep for the Study ofCopolymers and Blends" by Philippe Demont, et al.). The aim of suchprocesses, which will be examined in greater detail hereinafter, is tounderstand the behavior of materials by studying the relaxations andinternal motions which take place in order to optimize their mechanical,electrical, magnetic, etc., performances.

More generally speaking, the recovery process of a system applies to thephenomenon of recovering its initial state, after the application of adeformation has taken the system out of equilibrium. The recoveryprocess is stimulated by a (linear) temperature increase, or can occurisothermally over time. Relaxation phenomena in materials duringrecovery are the results of internal motions due to disturbances eitherof a mechanical, electrical, magnetic or electromagnetic nature.Materials processed in industry have physical properties which depend onthe ability to have local motion within the internal structureirrespective of whether this motion occurs at the level of themolecules, the atoms, or the macromolecules (for polymeric materials),or at the sub-atomic level. Deformation at one level or the otherdepends upon the type of excitation field involved to bring the materialout of its equilibrium state at a given temperature.

Essentially, three types of methods for studying relaxation phenomenaand resonances can be distinguished: (1) resonance methods, (2) dampinganalysis methods and (3) heat stimulated methods. In the resonancemethod, the material is subjected to a periodic excitation at a fixedfrequency of a mechanical, electrical or magnetic nature at a determinedtemperature and fixed pressure. The periodic excitation frequency can beadjusted to enable the determination of the resonance frequency for thistemperature and pressure. The frequency of resonance corresponds to thefrequency of the internal motion occurring under these conditions. Analternative method, which is frequently used, consists in subjecting thematerial to an excitation at a determined frequency and programming avariation in temperature. When the temperature reaches a level capableof allowing the internal movements sought to be characterized, aresonance peak for the selected excitation frequency is observed. It ispossible to operate at various (fixed) frequencies and thus analyze thedependence between frequency and temperature which provides access tothe mechanism responsible for internal motion under investigation.

In many instances, the internal motion is kinetically controlled, andthe variation in the resonance peak frequency (fm) varies with themaximum temperature of the peak Tm, and the results are often collectedas the 1n (fm) versus 1/Tm, a so-called Arrhenius diagram (Tm is indegrees Kelvin and 1n is the natural logarithm). The linearity of theArrhenius line is indicative of an activated phenomenon. The slope ofthe straight line in the Arrhenius diagram is related to the activationenthalpy of the process due to internal motions and the intercept isproportional to the activation entropy, i.e., to the jump frequencybetween the activated states allowing motion. By determining the valuesof the entropy and enthalpy, one can determine the origin of themovements occurring inside the material irrespective of their origin,whether it is viscous, atomic or sub-atomic. Mechanical deformationfields allow movements of the viscous type to occur in the material (sodoes ultrasonic excitation) and electrical fields (voltages) applied tothe material allow the study of motions related to the electronicinteractions between the atoms inside the material. The sub-atomicmovements are delocalized by applying electromagnetic excitations.

In the characterization processes which are the subject of thisinvention, the temperature program is always the same regardless of theorigin of the excitation, and consists of exciting the material at aparticular temperature, then quenching it, interrupted by partialisothermal relaxation if necessary, and finally heating it up linearlyto "develop" the response to the excitation stage during a thermallystimulated return to equilibrium.

The analysis methods using damping in the material consist in theapplication of a deformation of the material for a given length of timeat a given temperature, cutting off the source of the excitation andanalyzing the return to equilibrium (recovery curve) at that temperatureby recording the freely oscillating damping curve. The equation of therecovery curve gives direct access to the damping factor at thatcorresponding temperature. The frequency of the oscillation and thedamping factor relate to internal friction, and provide the relaxationtime at the corresponding temperature. The frequency of oscillation andthe damping factor vary with the temperature at which the material isbeing deformed. This enables the determination of the damping factor atdifferent frequencies and different temperatures. As above, the originof the internal motions may be found by studying the correspondingArrhenius diagrams in plots of log of

    1n (fm) vs. 1/Tm

The so-called "thermal stimulated" methods comprise purely calorimetricmethods and methods combining the influence of temperature and a"stimulant" variable which may be a mechanical, electrical or anelectromagnetic variable. Differential scanning calorimetry (DSC)consists in comparing the calorific energy flux supplied to twocrucibles located in the same thermostatic atmosphere, a device in whichone of the two crucibles contains the material to be analyzed. Thetemperature in the chamber may be programmed to increase, decrease, orto stay constant (isothermal mode). In a DSC the calorimeter is servoregulated in such a way that the temperature of the two crucibles isexactly the same. The variable energy flux supplied or subtracted fromthe crucibles is recorded as the temperature of the cell varies, or as afunction of time under isothermal conditions. Differential ThermalAnalysis (DTA) is a slight variant of this microcalorimetric DSCprocess, for which the fine difference in the temperature between thetwo crucibles is recorded as a function of the cell temperature. Thedifference in the temperature between the two crucibles changes whenthere is an alteration in the physical structure or in the physicaland/or chemical structure resulting in the variation in enthalpy withinthe material. In a DSC analysis, the energy flow differential tomaintain the two pans at the same temperature is recorded, and a peak isobserved when there is a modification in the thermodynamic state of thematerial. The peak characteristics relate to the state of the material,and transcribe the extent of internal motion and local reorganization,for instance due to molecular relaxations. Differential scanningcalorimetry is a rapid and streamlined method of determining phasetransitions in materials, for example in order to determine fusion andsolidification temperatures, and the glass transition temperature in thecase of the amorphous phase of non-crystalline or semi-crystallinematerials. It should be noted that in this characterization technique,temperature essentially plays two roles, that of stimulator bycontributing thermal energy capable of initiating activated internalmotions, and that of sensor, by comparative measurements of thetemperature or the flow of energy of the two crucibles, one containingthe material to be characterized.

One variant of this process consists in obtaining calorific heatcapacity curves as a function of the temperature at differentatmospheric pressures.

Atmospheric pressure plays an important role with respect to thekinetics of relaxation phenomena. It is presently known that an increasein pressure is accompanied by a restriction of internal movements, whichis observed in differential microcalorimetry by an increase in thetemperature at which the internal movements are released during athermal analysis. Apparatus currently marketed enable microcalorimetrycurves to be obtained at pressurized atmospheres. The pressure remainsconstant during the heating or cooling cycle of these analyses. It isone of the characteristics of the present invention to provide means tosubmit the crucibles and their content to a pressure history treatmentto enable the fine characterization of internal motions inside thematerial under investigation.

A further important type of analytical instruments for measuringinternal movements in materials by the thermal-stimulated effect isdescribed in the works of several authors, and concernsthermal-stimulated current techniques (TSC), and thermal-stimulatedcreep techniques (TSCR). These techniques are relatively original withrespect to the previously described techniques. In these techniquestemperature plays the role of developer while the external variablesimposed during the excitation stage play the role of "marker".

In a variant of the process, described in further detail hereinafter,temperature also plays the role of "filter" for the relaxation times;this is the "thermal-windowing" filtering method. The aim of theexcitation, in the form of a mechanical, electrical or magnetic field,etc., imposed on the material at a given temperature, is to induceorientation, or more generally to cause an imbalance in the system, bythe effect of the field on the free activation energy value. The fieldintensity imposed remains fixed for a given time, the time for the newstate of equilibrium to establish itself, and the temperature is loweredvery quickly (tempering) to a temperature at which the new thermodynamicstate of the material is no longer able to modify itself, for kineticreasons; consequently a "frozen-in picture" of the state obtained athigh temperature is produced. Analysis by the thermal stimulated effectconsists in suppressing the field at low temperatures and reheating thematerial, which is now free of all stresses, and in so doing freeing upthe internal motions which are thermally activated to allow their returnto equilibrium. The kinetics for the return to equilibrium, induced bythe temperature, can be analyzed quantitatively and is a function of theprocessing parameters of the material and its chemical structure. It isalso a function of the morphology.

The thermal stimulated effect reveals all the relaxation modes occurringin a global manner. If the local motions inside the material are notsimple in the sense of a pure relaxation of the Debye type, or whenthere is a large degree of interactive coupling between the relaxationmodes responsible for the global response of the material, it is thenvery difficult to attribute to the recovery curve any particular localmotion occurring in the material. Since the entire response of thematerial to a given excitation is global, it is generally essential todeconvolute the global response and define the relaxation timedistribution, corresponding by analogy to different coupled resonators.The coupling between the elementary modes of relaxation is subject to aspecific kinetics, itself a function of structural, chemical andmorphological parameters. The description of the elementary modes, theirthermo-kinetic characteristics (activation energy and entropy) and thedescription of the coupling is essential for understanding themacroscopic properties of materials. The TSC (thermally stimulatedcurrent) and TSCR (thermal stimulated creep processes) are thermalstimulated techniques which use the application of a field, eitherelectrical (for TSC) or mechanical (for TSCR) at a given temperature inorder to orient the dipoles in the material (TSC) or the chain segments(TSCR), with the aim of disclosing their individual existence whenheated up in a controlled manner after cooling, and after theapplication of the field has been removed.

The two techniques, TSC and TSCR, have been described in the literature(for the TSC: Chapter 10 in the ACS Book Polymer Characterization."Characterization of Polymers by Thermally Stimulated Current Analysisand Relaxation Map Analysis Spectroscopy, by J.P. Ibar, et al., PolymerCharacterization Advances in Chemistry Series No. 227, Edited by ClaraD. Craver and Theodore Provder; and for the TSCR: Chapter in the ACSBook Polymer Characterization, "Thermally Stimulated Creep for the Studyof Copolymers and Blends" by Philippe Demont, et al.).

The principle of the thermal stimulated windowing technique issummarized herewith. The technique has been used a great deal by thescientists of the Laboratory of Physique des Solides in Toulouse,France. These researchers, headed by Professor Lacabanne, concentratedon the application of the thermal windowing method with the aim ofisolating one by one the individual relaxations making up a cooperativecomplex spectrum. The method consists in applying an excitation field(electrical or mechanical) to induce orientation in the material at aselected temperature of excitation T_(p). The temperature issubsequently lowered by a few degrees, with the field still applied. Atthat temperature T_(d), the excitation field is then removed and thematerial is free to return to its state of equilibrium at thistemperature T_(d). However, it can only do so for a small time t_(d) andtherefore the material cannot relax completely at T_(d), and theremaining orientation induced in the material is then frozen in byquenching to a very low temperature T_(o). The subsequent reheating at acontrolled heating speed, discloses the elementary kinetics of therelaxation mode isolated in the window temperature range (T_(p) -T_(d)).The curve obtained during this recovery stage at a constant rate ofheating is of a Debye nature, which may be analyzed directly andquantitatively according to the Arrhenius formulation to determine theactivation enthalpy and activation entropy parameters for this isolateddeconvoluted elementary relaxation. By changing the value of T_(p)around the global temperature peak observed in either TSC or TSCR, allthe relaxation modes co-operating in an interactive manner andcontributing to the global response observed without thermal windowingcan be isolated one by one. This represents the description of the priorart according to the processes described as thermal stimulatedprocesses.

However, these known methods for analyzing and characterizing materialsby the thermal stimulated effect have major negative drawbacks: themethod using thermal stimulated current cannot be applied to conductorsor semiconductor materials for which the electrical resistance issmaller than 10φ ohms/meter of thickness; the method using thermalstimulated creep is not easy to apply to pasty or liquid materials anddoes not allow temperatures close to the fusion point of the materialsto be reached; and there is no simple correlation between thedistribution spectra for the relaxation times obtained by TSC and TSCRanalysis. This is a major drawback which casts a doubt on the validityof the results obtained by these techniques. The relationship betweenthe mechanical and electrical spectrum of relaxation appears to becomplex. In addition, the thermal stimulated method presented in theprior art appears to disturb the structural state of the sample owing tothe very nature of the experiment itself: the TSC or TSCR methodsconsist in applying an electrical or mechanical field at a temperatureT_(p) in the vicinity of the temperature at which the internal motionsoccur. The effect of bringing the material to this temperature T_(p)enables the latter to relax from its internal stresses, if there are anypresent, or to modify its morphology, if it is capable of crystallizing,or even modifying its degree of curing for curable materials andthermoset resins. It is therefore clear that thermal stimulatedprocesses are restricted to the study of internal motions undisturbed bymorphological changes at the analysis temperature T_(p).

The main disadvantage of differential microcalorimetry or ofdifferential thermal analysis is that the instrument response is aglobal response which integrates the co-operative plurality of internalrelaxations. A further main disadvantage is the low sensitivity indetecting "secondary" internal movements for which the activationenthalpy is low. Finally, this technique also presents difficulties,especially a lack of sensitivity, in studying certain phenomena such asthe orientation of plastic materials or the physical aging phenomena.For instance, it is not rare to observe great variations in themechanical properties of plastic materials and not to lack such evidenceof any difference on the basis of the corresponding traces in DSCanalyses. Differential microcalorimetry appears not to be very sensitiveto internal stresses relaxed kinetically during physical aging.

Another major disadvantage of the thermal stimulated processes describedin the prior art is that the sample must be changed for each temperatureT_(p) when the object of the analysis is to study physical aging orinternal stresses. This results in a long and expensive analysisprocess. In the prior art, a technician using the TSC analysis cell orTSCR analysis cell must prepare a variety of samples and introduce themin succession one after the other. The thermal windowing experiments arethen run according to the previous description and a new sample has tobe entered into the chamber for each T_(p) since the sample which hasbeen analyzed has lost its initial condition, which is what is beingstudied. The above procedure is repeated for each excitation temperatureT_(p) with a new sample until the complete relaxation spectrum isobtained. This method of analysis for isolating simple modes inmaterials having internal stresses requires a large number of samplesand a great deal of labor.

Yet another major disadvantage of the prior art is that the TSC and TSCRcells are different and the two techniques cannot be used simultaneouslyon the same sample.

SUMMARY OF THE INVENTION

It is one object of the present invention to remedy the aforementionedproblems and disadvantages by providing a means for analyzing themechanical and electrical spectrum for a given sample.

It is another object of this invention to add a variable to the existingprior art in order to remedy drawbacks of prior art processes and/orapparatus by using windowing processes which do not alter the morphologythe way thermal-windowing does, as will be explained hereafter. In otherwords, it is a characteristic of the present invention to describe anexcitation field profile which rheologically freezes the material at aconstant given temperature instead of changing the temperature in orderto create the window necessary to induce the filtering process (thermalwindowing).

It is yet another object of this invention to provide means forcharacterizing several samples simultaneously in order to allow thetechnique to adapt to the situation of a change in the internalstructure of the material with the change of temperature of excitationT_(p).

Even another object of the present invention is to overcome thedisadvantages of the known processes described in the prior art asthermal stimulated processes. Here, the invention proposes to create anefficient means for implementing a process which can be applied in arelatively general manner to a large number of materials and to a largenumber of transitions in materials enabling the analysis to be carriedout either to characterize mechanical, electrical, magnetic orelectromagnetic transitions. To this end, the invention concerns a meansfor implementing a process when at least two coupled excitation fieldsare applied to the sample of material analyzed.

In this invention, the excitation fields are selected from the groupconsisting of electrical excitation fields (ac or dc voltage),mechanical excitation fields (hydrostatic pressure, force, shear stress,oscillating or static), magnetic excitation fields, or electromagneticexcitation fields (ac or dc). Moreover, the variables which are used asoutput to characterize the resonance and relaxation behavior of thematerial are chosen from among the current, strain rate, the stressrate, and the energy flux differential to keep two crucibles at the sametemperature.

In one embodiment of this, an apparatus is provided which is designed toanalyze relaxation spectra in materials. This apparatus comprises: (a)means for varying the temperature of at least one sample of materialaccording to a predetermined temperature program; (b) means for applyingto the sample at least two coupled excitation fields simultaneously withsaid temperature program; (c) means for varying the excitation fieldsaccording to a predetermined excitation program co-extensive in timewith said temperature program; and (d) means for measuring at least onesample parameter representative of the relaxation behavior of the sampleduring a portion of the temperature program.

If the means for implementing this invention employs the use of a microcalorimetric device such as a DTA or a DSC, the special characteristicsconsist in detecting the changes between at least two samples (one beingused as a reference) which are assigned constantly the same temperaturebut which are subjected to two thermal pressures. Here, the pressure inthe crucibles, chamber changes in time according to a predeterminedprogram.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIGS. 1A, 1B and 1C are representative curves of a temperature program,a first excitation field P(t) and a second excitation field Q(t),respectively.

FIG. 2 is a simplified schematic diagram of an apparatus for practicingthe process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Analysis methods based on resonance and internal damping are "global"methods which do not enable elementary relaxation modes to be isolatedone by one, and very often the apparent activation energy which isobtained from the Arrhenius diagrams (1n fm vs. 1/Tm) is frequently toolarge to be realistic, which reveals the fact that the internal motionsobserved in a global peak are in fact coupled and that the response ofthe apparatus results from a cooperative coupling between a plurality ofrelaxations acting globally. This implies a major drawback of theresonance and damping methods in their attempt to give an interpretationto the origin of internal motion. It should be mentioned that thethermal analysis processes described as prior art in the literature(TSC, TSCR) are capable of reconstructing the global response of thematerial from the thermal windowed experiments, and thereforecalculating the resonance curves and damping characteristics of thematerial, i.e., providing the same output as the more traditionalthermal analysis equipment. This is why this invention is capable ofproviding both the spectra of relaxation of a given material, and thecharacteristic resonance at any given frequency or temperature, bycalculation from the spectrum of relaxation.

According to one of the characteristics of the invention, a means isprovided for varying the temperature (T) according to a predeterminedprogram which may be divided into several intervals, for instance fivezones--Z1, Z2, Z3, Z4, Z5, as shown in FIG. 1A. Thus, during theanalysis, the temperature evolves in a manner which is programmed, forinstance by means of equipment capable of PID controls.

Any suitable temperature adjusting means can be employed when practicingthis invention. One example of a specific temperature varying means isillustrated in FIG. 2. The embodiment illustrated therein will bediscussed later.

During this variation of the temperature, the material sample issubjected to the action of at least two excitation fields P(t) and Q(t)shown clearly in FIG. 1B and FIG. 1C. The variation of P(t) and Q(t) iscoupled and the coupling depends on the transition characteristics thatthe user has selected to analyze. The excitation fields vary with time tin a programmed manner.

In each zone, Z1 to Z5, the evolution of each of the excitation fieldsP(t) and Q(t) is represented by the corresponding curve sections in eachzone--P1(t), Q1(t): P2(t), Q2(t); P3(t), Q3(t); P4(t), Q4(t); P5(t),Q5(t). The different curve sections may be continuous or discontinuousdepending on the material analyzed and the nature of the analysis. Thetype of the excitation fields applied to the sample may be eitherelectrical, magnetic, electromagnetic or mechanical. The force exertedon the material might be a magnetic, mechanical or a hydrostatic force.The selection of the type of forces and fields depends on the nature ofthe transition which is to be characterized. The excitation fields arecoupled in order to isolate the elementary Debye modes of relaxationwhich contribute to the global response.

In a further embodiment of the temperature variation according to thepresent invention, a means is provided for varying the temperature as afunction of time according to a program similar to those described inthe thermal stimulated methods TSC and TSCR. Thus, the sample ofmaterial analyzed by the apparatus disclosed in the present invention isheated to an excitation temperature T_(p) at which the sample remainsfor a length of time t_(p). Subsequently, the apparatus is designed suchthat this temperature can be lowered by a few degrees in order to arriveat the depolarization temperature T_(d). The sample remains at thistemperature for the duration of time represented in FIG. 1A by zone Z2.At the end of this recovery period at T_(d), the sample undergoesthermal quenching in order to lower its temperature to the freezingtemperature T_(o), and from this temperature a linear variation oftemperature T=f(t) is applied. This temperature profile would be acharacteristic of the invention in FIG. 1A, according to this embodimentof the invention.

The present invention, differs from the prior art in that it provides,among other things, a means for superimposing the effect of at least twoexcitation fields as the temperature variation program changes. Theobjective here is to create a filtering of the various relaxation modesby means other than pure thermal-windowing effects. Variables which arecapable of individually modifying the recovery kinetics associated withlocal internal movements in the material can be coupled in the processdescribed here to create the desired filtering effect.

Any suitable excitation field application means can be employed whenpracticing this invention. One example of such a means is illustrated inFIG. 2. The embodiment therein will be discussed later.

Owing to the coupling between several types of excitation modes and theresulting effect it has on the spectrum of relaxation, the presentinvention enables the understanding and the decoupling of theinteractions between local movement occurring in the material, such asdipolar relaxations, and the global movements, such as the mechanicalinduced or viscous relaxations. This invention makes it possible tocharacterize such a coupling between the global aspect of thedeformation and the local aspect. For example, in one embodiment of theinvention, a means is provided for applying an electrical voltage fieldto a material at a given temperature which is coupled to a mechanicalfield applied to the material at the same time. The mechanical variablemay be either a hydrostatic pressure or a stress tensor. The variable,as measured during recovery, may either be the electric current producedby the material during heating or in isothermal condition, or the straindisplacement as resulting from recovering the effect of deforming thematerial at T_(p), or both of these at the same time. Coupling betweenapplied excitation fields enables one to sort out the origin and thedifferences between the dielectric relaxation resulting from electricalmotions and the mechanical relaxation due to viscous motions. Thecoupling laws between these internal motions due to either viscouscauses and electronic interactive causes can thus be determinedaccording to this characteristic of the invention. The excitationprofiles P(t), Q(t) describe how to apply the excitation fields such asto bring the material out of equilibrium, and back to equilibrium as afunction of T. The excitation profile of the fields may be identical ordifferent so as to demonstrate one or another particular characteristicof the recovery kinetics. This explains why there are many types ofprofiles for the two fields variations P(t) and Q(t). Each profile ofP(t) has to be programmed in and coupled with the program used for Q(t)in order to determine an excitation history, which then enablesdecoupling of both dielectric and mechanical relaxation giving rise to aglobal answer in the material.

In another embodiment of the invention, a means is provided for rangingthe pressure in the measuring chamber of a TSC or TSCR analysisapparatus according to a program. The pressure is coupled either withthe voltage field (in the case of TSC) or with a force field (in thecase of stress of TSCR). In the case of the application of anelectromagnetic or magnetic field to excite the subatomic structure ofmagnetic materials, the pressure will be coupled with the magnetic fielditself. In the particular case of coupling pressure effect with anotherfield, the pressure plays a role identical to temperature, in particularin the creation of a windowing effect (P_(d) -P_(p)) with the aim ofisolating rheologically simple relaxation modes. For instance, at T_(d),temperature of partial recovery, the pressure may be increased to delaythe recovery of a given set of relaxation times influenced by the effectof both pressure and temperature. Release of the pressure, still atT_(d), results in the recovery of the relaxation modes which have notyet relaxed due to the effect of pressure. In the case of an electricalfield coupled with pressure, if the motion of the dipoles activated bythe excitation voltage is influenced by a pressure effect, thepressure/voltage field coupling enables one to obtain the fullrelaxation spectrum in a much more rapid manner than for a classicalnormal TSC analysis, as described by the prior art. In other words, the"thermal windowing" may be carried out by other means than lowering thetemperature. In this case here, the window is created by a pressureeffect which offers the additional advantages of being fairly easilyimplemented, and also the window width can be very small, resulting inan increase resolution to resolve the elementary peaks.

In the above illustration of a preferred embodiment of the invention,hydrostatic pressure is shown as a variable capable of stimulating achange of state in the material around a phase transition, but theapplication of a mechanical, electrical or electromagnetic vibration mayserve the same objectives. The application of a vibration to a materialinduces changes in the value of the phase transition temperature, due tomodification of the internal state of the material. For instance, thetransition temperature increases as the vibration frequency increases (aphenomenon well known to material scientists and rheologists workingwith relaxation phenomena). The phenomenon of increasing the temperatureat which a transition occurs for a given material is equivalent to alowering of the temperature with respect to the transition temperature.

In other words, by changing the position of the temperature of the phasetransition at a given temperature, by vibrational means, one is able tochange the window width between the temperature of the test and thetransition temperature under investigation. For activated phenomena, anincrease in the vibration frequency between two activated levels isequivalent to a decrease in temperature.

Again, this effect may be used to define the "windows" similar to thosecreated with the thermal windowing method used by the prior art.Coupling thermal windowing effects (which are created by pure changes oftemperature) and "frequency or pressure simulated" windows (created bythe action of a vibrating field or by the effect of changing thehydrostatic pressure) enables one to characterize the local motions withrespect to their origin, whether it be of viscous or electronicinteractive nature.

It should be noted that the nature of the vibration applied during theexcitation state (either at temperature T_(p) or during the recovery atT_(d)) may be identical or different from the nature of the staticfield, applied in conjunction to it in order to create the coupling, andthat the detecting variable during the sensing stage (during theprogrammed rise in temperature to reveal a relaxation mode in therecovery zone) may be of the same nature as the vibratory variable orthe static field. For example, it is possible, in another embodiment ofthis invention, to use coupling between a mechanical field (hydrostaticpressure or a shear stress applied during a time t_(p) at temperatureT_(p)) with a vibratory field of electrical nature or of electromagneticnature (with a predetermined frequency and amplitude of vibration), thatvibratory excitation being applied at T_(p) or at T_(d) for a programmedtime. The recovery curve may be studied either with electric variables(in such a case a measurement of the depolarization current isperformed), or with a mechanical variable (variation of the strain andstrain rate during recovery), or lastly with a purely thermalmeasurement (measurement of the heat capacity changes during recovery).

In a particularly important embodiment of the invention, heat sensingmeans such as those used in a DSC or a DTA are used to detect motionsduring the recovery stage, after an initial excitation stage whichcomprises thermal and pressure windowing to filter out singularrelaxation modes. The cell chamber which includes the samples to beanalyzed is divided into two compartments, one at pressure P±, the otherat pressure P≧. The two compartments are strictly at the sametemperature irrespective of the temperature program T in FIG. 1A, or thepressure in each compartment, whether this is during the excitation orthe recovery phase. In a particular embodiment of the above arrangement,only two crucibles are located in each compartment, one of the cruciblesin each compartment containing a sample of the material to be analyzed.

It is believed that those skilled in the art will understand how toadapt a DSC or DTA apparatus as described in the forgoing paragraph and,accordingly, an illustration of such a modified apparatus is believedsuperfluous.

In a variant of the previous embodiment, the compartments may contain aplurality of crucibles, each containing a sample of the material to beanalyzed in addition to a control reference sample. This configurationis particularly suitable for studying physical aging phenomena andcuring or crosslinking or crystallization phenomena, or for studying thestate of internal stresses in the material. Note that in this embodimentof the invention, a single run will provide the measurement of severalsamples at once and submit it to the same temperature variation.

The rate of change of the microcalorimetric differentials between theseveral samples and between the crucibles are automatically recordedregardless of the compartment they are in and the temperature orpressure which is programmed to vary. The temperature and pressurevariations inside the cell chambers are programmed by a computer tocreate windowing effects which make it possible to separate out thesingular relaxation modes, provided that the relation occurring by thechange of temperature or pressure result in a modification in the heatcapacity or the enthalpy of the material. The microcalories supplied tothe crucibles may be compared for the crucibles located inside the samecompartment or for crucibles containing samples of identical origin butlocated in two different compartments and therefore at differentpressures. The analysis of the enthalpy difference leads to thecharacterization of the distribution of enthalpies attributed to aspectrum of relaxation modes.

In another embodiment of the invention, a means can be provided forprogramming the pressure in a specific way to rapidly study the kineticcharacteristics of a pressure sensitive phase transition, such as theglass transition temperature of glass forming materials. The action ofhydrostatic pressure on the material may be used to "create" at will atransition effect, since the transition itself occurs at a highertemperature if the pressure is suddenly increased in the chamber. Thesample, which is slowly heated up, is subjected to rapid pressurization(simulating quenching across the transition under investigation),resulting in states across the transition temperature, and subsequentlydepressurized at a controlled rate in order to analyze the kinetic curveof the change of state during the return to equilibrium, since theeffect of relaxing the pressure will be to cross the transition in theother direction, giving an opportunity to record the kinetic changesoccurring during this partial return to equilibrium. The temperaturechanges during that process can be slow enough to be considered constantand therefore the process can correspond to the study of a recoveryreturn to equilibrium under isothermal conditions. This means forpressurizing and relaxing may be performed a number of times during theslow increase in temperature in the chamber, thus providing a series ofkinetic relaxation curves which can be analyzed with the classical toolsof rheology and relaxation kinetics.

In a further embodiment of the invention, for coupling a mechanicalfield and a hydrostatic pressure, a means is provided for introducinginto a cell a plurality of samples to be compared, at the same time, inorder to be subjected simultaneously to the same pressurizing programsand the temperature variation programs. The responses obtainedsimultaneously for the various samples during recovery enable thedifferences existing initially in the materials to be compared veryquickly and in a single operation. This is particularly useful forstudying the internal stresses set in an object, for which thesestresses vary from one point of the object to the other owing to moldingconditions. For example, in the process used to manufacture compactdiscs or optical discs by injection molding, it is important toeliminate internal stresses in the direction parallel to the readinglaser beam. It is thus of prime importance that the material propertiesdo not vary over time and from point to point in the radial direction.This particular embodiment of the invention can be used for thesimultaneous study of a plurality of samples in order to determine aninternal stress intensity curve.

In a still further embodiment of the invention, the excitation fieldapplied during the windowing process is electromagnetic or corpuscular,for example luminous or sub-radiating (X ray, gamma rays, UV radiationetc.). This excitation mode may be more suitable for the analysis ofthin layers of conductor or semi-conductor materials, such as in thecharacterization of the electronic behavior of the amorphous component,in particular for testing the structure of energy which the globalenergy is composed of.

One specific embodiment of a novel apparatus encompassed by thisinvention is illustrated in FIG. 2. The apparatus includes a cell basehaving two halves 2 and 3, which are designed to close together tocreate a chamber which encloses a sample holder (unnumbered), anelectrode 4 and the sample 1. The apparatus further includes a thermalenvironment control means. Examples of such control means include,without limitation, heaters, cooling jackets, PID controllers,temperature sensors and the like. The environment control means isrepresented by item 20.

The specific apparatus illustrated in FIG. 2 also includes a means forapplying to the sample at least two coupled excitation fields.Specifically, exciters 5 are provided to provide desired excitationfields to the sample 1. The exciters 5 may include, without limitation,a motor such as a stepper motor to produce a mechanical force (eitherstatic or periodic) on the sample, a voltage field power supply, and acomputer means 12 to monitor and control the exciters according todesired programs such as illustrated in FIGS. 1B and 1C.

This apparatus also includes sensors and conditioning circuitry 6 tomonitor strain on the sample induced by the force applied to it, or bythe variations in sample dimensions due to a change of temperature. Fourtypes of sensors are envisioned:

(1) a stress sensor to control the level of stress applied to thematerial;

(2) a pressure sensor to monitor and control the pressure inside thechamber;

(3) strain sensors, such as strain gauges or LVDTs; and

(4) charge sensors, such as an electrometer for measuring electriccurrent released by the material upon relaxation.

In a preferred embodiment of the invention, the sample is heated/cooledvia a gas (helium, nitrogen) in which it is immersed. The gas isenclosed in a closed chamber 11 which is heated, cooled and pressurizedby external means which persons skilled in the art would know how toimplement.

For instance, the whole configuration for the process can be viewed inFIG. 2. Specifically, sample 1 is positioned between two metallic holderplates which act as sample holder and electrode. This sample holder islocated inside a cylindrical furnace, heated by a coil. The sampleholder and the sample is placed in the axis of the cylindrical furnace.This furnace assembly is, itself, immersed in the bigger cell assemblywhich consists of a cylindrical tube rounded up in a cooling jacketfilled with a liquid such as water, liquid nitrogen, liquid helium, orother known refrigerant liquids provided from a source 7.

The cell assembly can be vacuumed if desired. In FIG. 2, the vacuumsystem includes roughing and diffusion pumps 9 and 10, respectively,manifold 8, valves 18 and 19, vacuum pressure gauge 13, and flushinggauge 4. The cell assembly can also be filled with a conducting gas suchas pure-helium, argon, nitrogen, etc. provided by a source 11.

A system of valves (16, 17, 18, 19) is controlled by the computer means12 which opens or closes access to either a vacuum system (roughing pump9 and/or diffusion pump 10) or a gas tank filled with the conducting gasat the appropriate and predetermined pressure (monitored by sensor orgauge 14 and controlled by the computer 12).

All of the elements of the apparatus will be well-known to those skilledin the art and need not be described in great detail here.

One characteristic of the invention is that the same cell base, which isnormally used for Thermally Stimulated Current (TSC) measurement, canalso be used for the apparatus carrying out the means necessary for thepresent invention. This represents a clear advantage of this embodimentsince the same apparatus (with a few modifications) can be used forsimultaneous excitation and measurement of thermally stimulatedmechanical or electrical relaxation occurring in samples, in order tocharacterize their internal motion.

In FIG. 1A, the temperature profile is programmed via heating andcooling the conducting gas which constitutes the sample environment.When P(t) or Q(t) of FIG. 1A represents the pressure in the chamber, theconducting gas is put under pressure by external means known to a personskilled in the art. A preferred range for the pressure is between 1 barand 700 bars, and obviously, the thickness of the wall of the cell ismodified accordingly in order to accommodate the larger pressures.

In cases where the signal P(t) or Q(t) is a stress imposed on thesample, a load cell is located outside the cell assembly and iscontrolled by computer means 12 via the load cell sensor. The stressapplied can be a torsion, or a flat force, and the displacement can bemeasured by optical means (transmitted light), or by means of a LVDT,RVDT, Moire fringes, or capacitance.

The application of the stress on the sample holder can be done by meansof a stepper motor. The range of modulus preferred for the materialstudied here goes from 102 to 1011 dyn/cm². A preferred embodiment ofthe invention is to use the stepper motor in direct connection to thesample, which provides the stress on the sample. The strain induced bythe stress, and the change of strain during recovery (strain rate) arebest measured by means of an encoder disc with a dual laser counting. Apreferred embodiment for the encoder consists of an optically polishedmetal with laser etched markings with a predetermined resolution(typically μ). The encoder disk is preferably of minimum thermal mass,must be supported without friction, and must be able to withstand theheat generation during laser marking as well as the heat that may betransported from the heater area up through the central high modulusshaft to the strain detecting encoder disc.

The temperature of use is preferably between -150° C. to 350° C., (withliquid nitrogen as the coolant) or -260° C. to 0° C. (with liquidhelium).

The preferred current detector, when P(t) or Q(t) is a voltage, is anelectrometer capable of measuring current as low as 10-17 amperes up to10-9 amperes.

The excitation of the two coupled signals P(t) and Q(t) is donesimultaneously by programming before the experiment the ramps of thesignals at given predetermined intervals. Each of the signals(electrical power supply ac and/or dc) and stresses (torsion orflection, or compression) is electronically sensed, conditioned andcontrolled by PID means to conform to the programmed variation. Suchprocedure is known to persons skilled in the art of PID controls.

A computer is used to record the outputs from the sensors and sendsignal to the exciters. The data (current of polarization,depolarization, stress on the sample, strain (angular, longitudinal orvertical depending on the type of stress), and its derivative strainrate, are continuously computed and archived on the computer storagemedium. The analysis of the data is done as either strain rate versustemperature for the mechanical deformation, or current versustemperature for the electrical signal. The measurements are donesimultaneously. The direct outputs provide direct information on theresonance characteristics, either mechanical or electrical, of thematerial at the equivalent frequency of excitation.

The relaxation spectrum can be calculated by using the method ofthermal-windowing, as explained in the prior art, depending on thefunction T(t) in FIG. 1A.

When ac signals are used for P(t) and/or Q(t), the detecting devices canbe such that the ac response of the material is continuously comparedwith the ac excitation, in order to, during the analysis stage, obtainthe variation of the storage and loss moduli and dielectric constant.The use of ac signals during the excitation stage serves, however,another purpose, in the present invention, since its use is primarily toinduce windowing effects which will be revealed in the sensing stage,upon recovery when the sample is heated up linearly at the end of theexperiment. The use of ac signals is to serve as an additional windowingtechnique, in the sense described herein. The use of hydrostaticpressure serves the same purpose.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

That which is claimed is:
 1. An apparatus for analyzing relaxation spectra in materials, said apparatus comprises:(a) means for varying the temperature of at least one sample of a material according to a predetermined temperature program; (b) means for applying to the sample at least two coupled excitation fields simultaneously with said temperature program; (c) means for varying the excitation fields according to a predetermined excitation program co-extensive in time with said temperature program; and (d) means for measuring at least one sample parameter representative of the relaxation behavior of the sample during a portion of the temperature program.
 2. An apparatus as recited in claim 1 wherein said excitation field application means is designed to apply an excitation field selected from the group consisting of electric excitation fields, mechanical excitation fields, magnetic excitation fields and electromagnetic excitation fields.
 3. An apparatus as recited in claim 1 wherein said means for measuring at least one sample parameter is designed to measure temperature.
 4. An apparatus as recited in claim 3 wherein said means for measuring at least one sample parameter is selected from the group consisting of a micro calorimeter and a differential thermal analyzer.
 5. An apparatus as recited in claim 3 wherein said apparatus further comprises:(a) means for simultaneously varying the temperature of at least two samples according to said temperature program; (b) means for maintaining said samples at the same temperature but at different pressures; and (c) means for performing differential calorimetry by measuring simultaneously the temperature difference between said samples and a reference temperature.
 6. An apparatus as recited in claim 3 wherein said apparatus further comprises means for varying the pressure over time according to a predetermined program.
 7. An apparatus as recited in claim 1 wherein said excitation field application means is designed to apply oscillatory fields. 